The present invention relates to a mixing circuit for use in a transmitter or a receiver which is of a synthesized type using transistors.
In order to obtain proper stability agaist variations of temperature and power supply voltage, a transistor amplifier stage generally has a bias applied to the base terminal through two resistors R.sub.B and R.sub.G, the resistor R.sub.B being connected at one end to the power supply B and at the other end to the base terminal, and the resistor R.sub.G being connected at one end to the base terminal and at the other end to ground, as shown in FIG. 1.
A conventional mixing circuit configuration is shown in FIG. 2 by way of example. As shown, the base of the transistor 4 of a mixing stage 1 which receives the output f.sub.1 of a first oscillator stage (not shown) is coupled to the emitter of the transistor 5 of a second stage 3 through a capacitor C.sub.c (2). Biasing resistors R.sub.B and R.sub.G are connected to each of transistors 4 and 5. Therefore, this configuration can be regarded as having the equivalent coupling circuit shown in FIG. 3. Thus the voltage E.sub.O applied to the base of the mixing stage 1 can be expressed by the following equation: ##EQU1## Consequently, the voltage E.sub.O approaches the value of e(f.sub.2) in the high frequency range and drops in the low frequency range since the value of 1/.omega.C.sub.c increases. Accordingly, the fundamental component of the waveform of e(f.sub.2) is attenuated but it harmonics are largely unaffected. This fact applies to the high frequency components created by the distortion of the waveform in the second oscillator stage 3. In other words, the coupling using the capacitor C.sub.c (2) functions, so to speak, as a high-pass filter. As a result, the conventional circuit is subject to noise interference caused by unnecessary radiated beats.